This invention relates to a method for operating a high pressure discharge lamp wherein an alternating lamp current is supplied to the high pressure discharge lamp.
The invention also relates to a circuit arrangement for operating a high pressure discharge lamp comprising
input terminals (K1, K2) for connection to a supply voltage source, PA1 means, coupled to the input terminals, for supplying an alternating lamp current to the high pressure discharge lamp.
Such a method and such a circuit arrangement are known from the U.S. Pat. No. 4,485,434. It has been found that AC operation of high pressure discharge lamps with a low frequency alternating lamp current prevents a rapid erosion of the electrodes of the high pressure discharge lamp (further also referred to as the lamp) and allows operation of the lamp with a relatively high efficacy.
A problem associated with the operation of this type of lamp is that depending on the temperature of the electrode and the condition of the surface of the electrode, the discharge arc in the direct vicinity of the electrode is often not stable because the origin of the discharge are jumps from one spot on the surface of the electrode to the next. If the surface of the electrode is too cold, the discharge are is very thin in the immediate vicinity of the electrodes and therefore causes overheating of its origin on the surface of the electrode thereby creating micropikes. During operation the origin of the discharge are jumps between these micropikes, causing a flickering of the high pressure discharge lamp. Flickering can also be caused by too high an electrode temperature. Under such conditions the perpetual displacement and evaporation of electrode material are the cause of instabilities in the discharge arc. In the case where a high pressure discharge lamp is operated with an AC current, each electrode of the lamp alternatingly functions as a cathode and as an anode during successive half periods of the lamp current. During these half periods the electrode is said to be in the cathodic phase and the anodic phase respectively. Electrode material, that is removed from the electrode in the anodic phase, returns to the electrode as a stream of ions in the cathodic phase. These transport processes further complicate the behaviour of the electrode temperature during a period of the lamp current since the time dependency of the electrode temperature in the anodic phase differs from that in the cathodic phase. Because of this, the electrode temperature varies strongly over a period of the lamp current and the discharge arc originates from different places on the surface of the electrode during the anodic phase. In the cathodic phase, however, the origin of the discharge arc on the surface of the same electrode will be located arbitrarily in only one of these different places. This behaviour is particularly unacceptable when the high pressure discharge lamp is used in an optical application such as projection television. In such an application the distance between the electrodes needs to be very short since the discharge arc should approach a point light source. Because of the short distance between the electrodes, however, the fact that the discharge arc can originate from a different place on the electrode during every other cathodic phase causes instabilities through the whole discharge arc and therefore a very strong flickering.